Hello Chris

uMirror is not allowed to receive URL, only absolute path. Your case is quite similare to the example described in the documentation.

You just have to create a proxy method for read the Feed and save it in your disk, you can do it with few lines of code. I give you a simple example:

const string FilePathUmbracoBlog = "\\App_Data\\umirror\\Umbracoblog.xml";

[UMirrorProxy(FilePathUmbracoBlog)]
public static void UmbracoBlogRssProxy()
{

        // Read umbraco blog feed
        XDocument myXDocument = new XDocument();
        myXDocument = XDocument.Load("http://feeds.feedburner.com/umbracoblog");

        // Save the Xml 
        myXDocument.Save(HttpContext.Current.Server.MapPath(FilePathUmbracoBlog));

}

Then, in your umirror project, specify that you are using this method and try it:

Let me know, if you have any other question !

Regard